Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperature, thus requiring spark retard, which in turn reduced fuel efficiency.
The inventors herein have recognized that under some conditions, utilizing intake manifold pressure boosting in which valve overlap near top dead center (TDC) is present may be advantageous in reducing residuals. Further, such valve timing and pressure boosting, in combination with direct injection fueling (e.g., in-cylinder injection) may be used to reduce charge temperatures and reduce the tendency for auto ignition (knock), thus enabling improved torque output and fuel economy under selected conditions.
However, the inventors have also recognized that under some conditions during valve overlap in which both intake and exhaust valves are concurrently at least partially open near TDC (such as near the end of the exhaust stroke, beginning of intake stroke) and intake manifold pressure is higher than exhaust pressure (due to boosting), fresh charge that does not participate in combustion may flow through to the exhaust manifold. Thus, by in some cases, it may be beneficial to utilize direct fuel injection that begins after exhaust valve closed (to reduce the likelihood of unburned fuel passing through to the exhaust) along with rich combustion so that an overall exhaust mixture may be maintained near stoichiometry. In this way, improved performance along with reduce emissions may be achieved.